This invention relates to a method and apparatus for drawing an optical waveguide fiber from an optical waveguide prefrom with reduced diameter variations and reduced fiber bow.
Methods and devices for drawing optical waveguide fibers from optical waveguide preforms are well known in the art. The devices typically consist of a heat source for softening the preform, a fiber diameter measurement device, a unit for applying protective coatings to the fiber, and a fiber take up unit.
It has long been recognized that dimensional variations in optical waveguide fibers significantly impact optical properties. For example, it has been shown that a 3% relative variation in fiber diameter can result in an attenuation contribution of up to 0.8 dB in the first kilometer of length. Montierth, "Optical Fiber Drawing Techniques", Optical Spectra, pages 42-48, 43, October, 1978. As early as 1978, it was stated that fiber diameter variations would need to be reduced to a 3.sigma. range of .+-.1% or better to produce fibers which would compete with traditional copper twisted wire pairs in the telecommunications market. Id. at 43.
Another issue to which fiber drawing technology has been directed has been the improvement of fiber strength. For example, the cleanliness of the draw devices has been improved by the introduction of gas flows into the heat source or furnace. This gas flow prevents updrafts of ambient air from entering the furnace from the bottom of the furnace where the fiber exits the furnace. Such updrafts can carry particles into the vicinity of the softened portion of the preform. These particles can adhere to the softened preform or fiber and may result in weaker points in the fiber which may break below the required limit. The gas flow also flushes materials which may evolve from the furnace or heated preform. See, for example, Bailey U.S. Pat. No. 4,126,436.
However, the introduction of the gas causes diameter variations in the fiber if the gas is not uniformly heated when it reaches the tip of the preform from which fiber is drawn because of turbulence caused by the rapid non-uniform heating of the gas in the vicinity of the preform tip. One solution to this problem of non-uniform heating is the addition of a thin, cylindrically-shaped channel adjacent to the preform which causes the gas to be uniformly heated when it reaches the tip of the preform. See, for example, Bailey '436. Also, it has been shown that the presence in the furnace of a gas such as helium may stabilize the temperature at the tip of the preform from which the fiber is drawn. This stabilized temperature at the tip of the preform has been shown to reduce the diameter variation in the fiber. See, for example, Bailey U.S. Pat. No. 4,154,592.
We have found that differential cooling of a drawn fiber before the viscosity of the cladding layer of the fiber is high enough to substantially prevent differential stresses in the drawn fiber may cause the fiber to bend. This bending, or fiber "bow", causes difficulty when the fiber is spliced to other fibers, resulting in high loss splices which are detrimental to the overall performance of an optical fiber-based telecommunications system. The effects of bow are compounded in ribbon fiber applications where one ribbon array of fibers must be spliced to an opposing array.
We have also found that room air circulating around the fiber as it exits the draw furnace differentially cools the fiber. This causes a high frequency and random diameter oscillation; typically, this variation is less than that allowed in the specification for the fiber diameter. However, these diameter oscillations are of sufficient magnitude to hide an imperfection in the fiber known as an "airline". An airline is a hole in the fiber and is identified by a sudden change of small magnitude in fiber diameter.
We believe that bow results from differential cooling of the fiber before the viscosity of the cladding layer of the fiber is high enough to substantially prevent differential stresses in the drawn fiber. As a fiber cools, the cladding layer cools rapidly relative to the core region. This rapid cooling of the cladding layer induces high tensile stresses into the cladding layer of the fiber. Any differential cooling of the cladding layer before the viscosity is high enough to minimize differential stresses in the cladding layer will result in tensile stresses which are not uniformly distributed around the circumference of the fiber, thereby inducing bow.
During the drawing process, a boundary layer of helium (or other gas used in the furnace) forms adjacent to the fiber surface. This boundary layer will travel with the fiber through the exit of the furnace unless it is disrupted by other forces such as additional gases introduced intentionally or inadvertently to the furnace atmosphere. Since helium is an extremely efficient heat transfer medium, this boundary layer can provide substantially symmetric temperatures around the circumference of the fiber so long as the boundary layer remains intact. We believe that the disruption of the boundary layer by currents in the ambient atmosphere contributes to the differential cooling which can lead to fiber bow or diameter oscillations.
Van der Giessen et al. U.S. Pat. No. 4,763,427 discloses the use of nitrogen, argon, or oxygen to prevent thermally induced stresses in a fiber. These stresses increase the attenuation of the fiber and are caused by large temperature gradients when the fiber is cooled from about 1800.degree. C. to about 1200.degree. C. The gas is introduced into the furnace and is heated to approximately the temperature of the preform. As the fiber is drawn from the heated preform into an attached tube, the gas flows from the furnace into the tube. Additional gas is introduced into the tube at a flow rate to produce a laminar flow profile inside the tube. The length of the tube is dependent on draw speed as the fiber must be resident in the tube for at least 0.1 sec to achieve the desired affect. Col. 1, lines 54-65.
Japanese Patent Application No. 62-246,837 ("JPA '837") discloses the use of a tube at the fiber outlet end of a draw furnace to reduce the diameter variation of the fiber. Inert gas is introduced into the draw furnace. This inert gas is heated to a temperature close to that of the preform and flows out of the furnace into the tube as the fiber is drawn from the furnace into the tube. There is also a shutter provided between the furnace and the tube to prevent the heating of the tube by radiant heat from the furnace.
The tube in JPA '837 is equipped with a medium for cooling the inert gas flowing into the tube from the furnace. JPA '837 also discloses means for introducing additional inert gas directly into the tube. The inert gas in the tube is cooled such that the temperature difference between the inert gas exiting the bottom of the tube and the ambient atmosphere will be negligible. This is designed to prevent ambient air from entering the cooling tube, and therefore, the furnace. It is stated that the introduction of ambient air into the furnace causes turbulent flows in the vicinity of the tip of the preform from which fiber is drawn which will cause variation in the diameter of the fiber due to uneven temperature profiles within the turbulent flows.
Shang European Patent Application No. 0,321,182, published Jun. 21, 1989, discloses and claims a method o to decrease the temperature of a drawn fiber in a controlled manner to result in relatively low absorption losses in the fiber. In Shang the tubular recovery chamber is used to achieve this controlled decrease in temperature.
The tubular recovery chamber of Shang may be heated, or a gas at an elevated temperature may be introduced into the chamber. Col. 6, lines 50-53. The temperature at the exit of the chamber is about 200.degree. C. Col. 7, lines 8-11. Shang discloses a seal between the draw furnace and the recovery chamber to prevent the ingress of uncontrolled ambient air into the chamber adjacent to the furnace. Col. 6, lines 18-21. Shang also discloses the addition of additional gas into the furnace near the seal between the furnace and the recovery chamber. This gas stream will tend to disrupt any boundary layer which may have formed adjacent the fiber, and it is believed that this disruption will cause differential cooling of the fiber which could result in diameter variations or fiber bow. Also, Shang does not disclose nor suggest the use of any device to prevent the ingress of ambient air to the exit end of the recovery chamber.
Shang is directed toward reducing draw-induced absorption losses caused by broken bonds in the glass structure. The recovery chamber of Shang causes the fiber to be exposed to a temperature profile Which allows the broken bonds to reestablish prior to exposure to the ambient air. This prevents "freezing" the broken bonds into the glass structure of the fiber.
Various other fiber cooling devices have been disclosed. These devices are used to cool the fiber for the purpose of applying protective coatings. Claypoole et al. U.S. Pat. No. 4,208,200 discloses a liquid fiber cooler comprising an elongated chamber through which the fiber passes. The elongated chamber is surrounded by a coolant jacket which lowers the temperature of the cooling liquid in the container.
Miller U.S. Pat. No. 4,437,870 discloses a fiber cooler comprising an elongated tube through which the fiber passes and into which cool dry helium is introduced.
Darcangelo et al. U.S. Pat. No. 4,514,205 discloses a fiber cooler comprising an elongated tube through which the fiber passes. As in Miller, Darcangelo et al. discloses the introduction of cool dry helium into the elongated tube. However, Darcangelo et al. further discloses the use of a chamber containing a liquified gas with a coil submerged therein through which the cool dry helium is passed before being flowed into the elongated tube.
Claypoole et al., Miller, and Darcangelo et al. all disclose fiber cooling devices which are located after the diameter measurement device. These cooling devices are, therefore, unsuited for reducing diameter variations in the fiber.
Paek et al. U.S. Pat. No. 4,594,088 discloses a liquid fiber cooler located between a draw furnace and an apparatus for coating the drawn fiber. It appears that the liquid fiber cooler of Paek et al. is located at a position after the fiber has passed through a fiber diameter measurement device, although Paek et al. only explicitly locates the liquid fiber cooler between the furnace and the coating device. Paek et al. does not disclose or suggest the use of furnace gases for cooling the fiber. Also, Paek et al. is concerned with cooling the fiber to a temperature below 80.degree. C. prior to the application of protective coating materials.